Compositions and methods for increasing protein half-life in a serum

ABSTRACT

Novel antibodies, such as single domain antibodies (sdAbs), or fragments thereof that specifically bind a transferrin are described. Compositions, methods and systems for increasing the half-life of a target protein in a serum using an antibody or fragment thereof against a transferrin are described.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No. 14/903,156, filed Jan. 6, 2016, which is a Section 371 of International Application No. PCT/US14/45768, filed Jul. 8, 2014, which was published Jan. 15, 2015 under International Publication No. WO 2015/006337 A2, which claims priority to U.S. Provisional Patent Application No. 61/843,628, filed Jul. 8, 2013, the discloses of which are incorporated herein by reference.

REFERENCE TO SEQUENCE LISTING SUBMITTED ELECTRONICALLY

This application contains a sequence listing, which is submitted electronically via EFS-Web as an ASCII formatted sequence listing with a file name “689296-2U2 Sequence Listing,” creation date of May 14, 2019, and having a size of 40.9 kb. The sequence listing submitted via EFS-Web is part of the specification and is herein incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

Pharmacokinetics of a drug candidate is a critical parameter and often largely determines whether or not the drug candidate will be further developed into a drug or used for a therapeutic application. In particular, pharmacokinetic studies of protein- and peptide-based therapeutics, including antibodies, have demonstrated that such therapeutics have varying serum half lives, and those peptides and proteins with a short serum half-life, although having a promising therapeutic potential, are thus often unsuitable for further drug development. For example, albumin and gamma immunoglobulins (IgGs) are known to have very long serum half lives of up to 20 days [1]. In addition, the Fc domain of other IgGs can be engineered to alter their binding interactions to neonatal Fc receptor as a method of prolonging serum half-life [2]. However, many other natural human proteins, such as insulin, antibody fragments such as antigen binding fragments (Fabs) or single chain variable fragment (scFvs), and short peptides usually have much shorter serum half-lives ranging from minutes to approximately only 1 hour. An efficient, cost effective and safe way of extending serum half lives of proteins and peptides with short half-lives is therefore critical for these molecules to become therapeutic drugs, diagnostic tools, etc.

Several strategies have been developed in an effort to prolong the serum half-life of proteins and other peptidic molecules that are short-lived in serum to avoid clearance of such therapeutic or diagnostic proteins from circulation. Several technologies employed to facilitate serum half-life extension of proteins and peptidic molecules include conjugation with a chemical attachment such as polyethylene glycol (i.e. pegylation) [3], fusion to the Fc region of an antibody [4], and fusion to a protein naturally having a long serum half life, such as albumin [5]. Unfortunately, these technologies suffer from complications including complex manufacturing and characterization processes, low expression levels, and undesired functions of the generated molecules.

Another approach that has been more recently developed to extend the serum half-life of proteins and other peptidic molecules employs the use of antibody fragments against serum proteins. Albumin, due mainly to its high serum concentration and long serum half-life, has been the most selected target for this purpose. An isolated domain antibody against human albumin was shown to prolong the serum half-life of interferon (IFN)-α2b after the two molecules were fused at the genetic level and expressed as a fusion protein [6, 7]. The serum half-life of the newly generated fusion protein molecule is not only longer than that of IFN-α2b, but even longer than that of the fusion protein of albumin and IFN-α2b.

Heavy chain variable domains of camelid heavy chain antibodies (HCAbs), known as VHH or single domain antibodies (sdAbs), have also been exploited for this purpose. For example, a sdAb against human albumin was shown to extend the serum half-life of an anti-TNFα sdAb fragment from less than one hour to over two days [8].

Another serum protein with a long half life is transferrin. Transferrin is a plasma glycoprotein that transfers iron ions and has a serum concentration of approximately 3 g/L and serum half-life of 7-8 days. Thus, transferrin is an ideal fusion partner to extend the serum half life of peptidic molecules with unsatisfactory pharmacokinetics. Studies have shown that fusion to transferrin significantly extended the serum half-life of both glucagon-like peptide 1 (GLP1) [9] and acetylcholine receptor [10].

However, the use of antibodies and antibody fragments such as sdAbs against transferrin, for increasing the serum half-life of peptidic molecules has not been reported. New methods for increasing the serum half-life of proteins and for producing proteins with improved serum half-life, that are efficient, cost-effective, and produce such proteins in high yield would facilitate the development of novel protein-based diagnostics or therapeutics. Embodiments of the present invention relate to such methods.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to novel antibodies against transferrin, and particularly single domain antibodies (sdAbs) against transferrin. The present invention also relates to methods of using antibodies that specifically bind transferrin to increase the half-life of a target protein in the presence of the transferrin, novel fusion proteins comprising the target protein having an increased half-life in the presence of the transferrin, and compositions comprising the antibodies that specifically bind transferrin, or a fusion protein according to the invention.

In one general aspect, the present invention relates to an isolated antibody or fragment thereof that specifically binds a transferrin, the antibody or fragment thereof comprising one or more selected from the group consisting of:

-   -   (a) a complementarity determining region (CDR) 1 comprising an         amino acid sequence selected from the group consisting of         GSGFGINGVI (SEQ ID NO: 1); GSGFGVNGVI (SEQ ID NO: 2); GNVFTIAAMG         (SEQ ID NO: 3); GNVFTIAAMA (SEQ ID NO: 4); GNVFTIDAMG (SEQ ID         NO: 5); GSVFSIDAMG (SEQ ID NO: 6); GNVFGIDAVG (SEQ ID NO: 7);         GSIFSIKVMG (SEQ ID NO: 8); and GSIFPLNDMG (SEQ ID NO: 9);

(b) a CDR2 comprising an amino acid sequence selected from the group consisting of LIKSDGYTNYRESVKG (SEQ ID NO: 10); LIKSDGYTNYRESVRG (SEQ ID NO: 11); GITTGGSTNYADSVKG (SEQ ID NO: 12); GMTNGGKTNYADSVKG (SEQ ID NO: 13); AMTNAGSTNYADSVKG (SEQ ID NO: 14); ATTTSGSSTNYADSVKG (SEQ ID NO: 15); DITSGGSTDYSDSVKG (SEQ ID NO: 16); and TITRGGTTNYADSVKG (SEQ ID NO: 17); and

-   -   (c) a CDR3 comprising an amino acid sequence selected from the         group consisting of PGVP (SEQ ID NO: 18); VTKWAARVGGSAEYE (SEQ         ID NO: 19); RSKLIATINNPYDY (SEQ ID NO: 20); RSKLIARINNPYEY (SEQ         ID NO: 21); RPKQATLIRDDY (SEQ ID NO: 22); DLGCSGAGSCPDY (SEQ ID         NO: 23); and DNRVGGSY (SEQ ID NO: 24).

Preferably, the antibody or fragment thereof is a single domain antibody (sdAb). More preferably, the antibody or antibody fragment thereof is an sdAb comprising the amino acid sequence of

A60219: (SEQ ID NO: 26) QVQLVESGGGLVQAGGSLRLSCVASGSGFGINGVIWYRQAPGKQRELVAL IKSDGYTNYRESVKGRFTISRDDAKNTVWLQMNALEPEDTGVYYCKTPGV PFGQGTQVTVSS; A60401: (SEQ ID NO: 28) QVKLEESGGGLVQAGGSLRLSCEASGNVFTIAAMGWFRQAPGKERELVAG ITTGGSTNYADSVKGRFTISRDNAQNTMYLQMNSLRPEDTAAYSCNAVTK WAARVGGSAEYEWGQGTQVTVSS; A69449: (SEQ ID NO: 56) QVQLVESGGGVVQAGGSLRLSCVASGSVFSIDAMGWYRQAPGNQRELVAA MTNAGSTNYADSVKGRFTISRDNAENTVYLQMNSLKPEDTAVYYCNARSK LIARINNPYEWGQGTQVTVSS; A69433: (SEQ ID NO: 58) QVKLEESGGGLVQAGGSLRLSCVASGNVFGIDAVGWYRQAPGKQRELVAA TTTSGSSTNYADSVKGRFTISRDIAKNTVYLQMDSLKPEDTAVYYCYARP KQATLIRDDWGQGTQVTVSS; or A69447: (SEQ ID NO: 62) QVKLEESGGGSVQAGGSLRLSCTGSGSIFPLNDMGWYRQAPGKQRELVAT ITRGGTTNYADSVKGRFTISRDSNAKNTVYLQMNSLKVEDTAVYYCNMDN RVGGSYWGQGTQVTVSS.

The present invention also relates to a fusion protein comprising an antibody or fragment thereof according to embodiments of the present invention, a target protein, and optionally a linker, wherein the antibody or fragment thereof is fused to the carboxyl-terminus or amino-terminus of the target protein, and the linker optionally separates the antibody and the carboxyl-terminus or amino-terminus of the target protein.

The present invention also relates to a nucleic acid molecule comprising a cDNA or synthetic DNA encoding an antibody or fragment thereof according to embodiments of the present invention, or a nucleic acid encoding a fusion protein according to embodiments of the present invention, and related expression vectors and host cells.

In another general aspect, the present invention relates to a method for increasing the half-life of a target protein. According to embodiments of the invention, the method comprises:

-   -   (1) obtaining a fusion protein, wherein the fusion protein         comprises an antibody or fragment thereof that specifically         binds a transferrin, the target protein, and optionally a         linker, wherein the antibody or fragment thereof is fused to a         carboxyl-terminus or amino-terminus of the target protein, and         the linker optionally separates the antibody and the         carboxyl-terminus or amino-terminus of the target protein; and     -   (2) exposing the fusion protein to the transferrin to thereby         increase the half-life of the target protein in the fusion         protein compared to the target protein alone.

According to an embodiment of the present invention, the fusion protein is obtained by a method comprising:

-   -   (a) obtaining an expression vector encoding the fusion protein;     -   (b) introducing the expression vector into a cell to obtain a         recombinant cell;     -   (c) growing the recombinant cell under conditions to allow         expression of the fusion protein; and     -   (d) obtaining the fusion protein from the recombinant cell.

Another general aspect of the invention relates to a composition comprising an effective amount of a fusion protein, wherein the fusion protein comprises an antibody or fragment thereof that specifically binds a transferrin, a target protein, and optionally a linker, wherein the antibody or fragment thereof is fused to the carboxyl-terminus or amino-terminus of the target protein, and the linker optionally separates the antibody and the carboxyl-terminus or amino-terminus of the target protein. Preferably, the composition further comprises the transferrin.

The present invention also relates to a method comprising exposing a composition according to an embodiment of the present invention to the transferrin to thereby increase the half-life of the target protein in the fusion protein.

A further aspect of the present invention relates to a system for increasing the half-life of a target protein, the system comprising:

-   -   (1) an expression vector comprising a first nucleotide sequence         encoding an antibody or fragment thereof that specifically binds         a transferrin, and optionally a second nucleotide sequence         encoding a linker, wherein the first and second nucleotide         sequences are operably linked,     -   (2) a host cell, and     -   (3) the transferrin.

Other aspects, features and advantages of the invention will be apparent from the following disclosure, including the detailed description of the invention and its preferred embodiments and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings:

FIG. 1 is a graph showing the immune response of llama against human transferrin after being immunized with an antigen;

FIG. 2 is an alignment of the amino acid sequences of 19 isolated single domain antibodies (sdAbs) that bind human transferrin; complementarity determining regions (CDRs) and framework regions (FRs) are separated with a space; the sequences are numbered according to Kabat numbering; the sdAb sequences shown include A60219 (SEQ ID NO: 26), A13152 (SEQ ID NO: 30), A12722 (SEQ ID NO: 34), A12680 (SEQ ID NO: 36), A12690 (SEQ ID NO: 38), A13154 (SEQ ID NO: 40), A13146 (SEQ ID NO: 42), A13149 (SEQ ID NO: 44), A12666 (SEQ ID NO: 46), A12659 (SEQ ID NO: 48), A13355 (SEQ ID NO: 50), A12692 (SEQ ID NO: 52), A60401 (SEQ ID NO: 28), A13376 (SEQ ID NO: 32), A69476 (SEQ ID NO: 54), A69449 (SEQ ID NO: 56), A69433 (SEQ ID NO: 58), A69441 (SEQ ID NO: 60), and A69447 (SEQ ID NO: 62);

FIGS. 3A and 3B are images of an SDS polyacrylamide gel of purified sdAbs A60219, A60401, A69433, A69447, and A69449, isolated from Escherichia coli, the amino acid sequences of which are shown in FIG. 2;

FIGS. 4A and 4B show surface plasmon resonance (SPR) sensorgrams of transferrin sdAbs binding to human transferrin; FIG. 4A shows SPR sensorgrams of sdAb A60401 binding to human transferrin at varying concentrations of sdAb of 2 nM, 1 nM, 0.5 nM, 0.25 nM, and 0.125 nM, from the top to the bottom of the plot; FIG. 4B shows SPR sensorgrams of sdAb A60219 binding to human transferrin at varying concentrations of sdAb A60401 of 2 nM, 1 nM, 0.5 nM, 0.25 nM, and 0.125 nM, from the top to the bottom of the plot;

FIG. 5 is a chromatogram of size exclusion chromatography (SEC) analysis of transferrin sdAbs A60401 and A60219;

FIGS. 6A, 6B, 6C, 6D, and 6E show graphs of thermo-denaturation curves of transferrin sdAbs measured by circular dichroism (CD) at 202 nm, 208 nm, and 217 nm; FIG. 6A shows thermo-denaturation curves of sdAb A602019; FIG. 6B shows thermo-denaturation curves of sdAb A60401; FIG. 6C shows thermo-denaturation curves of sdAb A69433; FIG. 6D shows thermo-denaturation curves of sdAb A69447; and FIG. 6E shows thermo-denaturation curves of sdAb A60449; and

FIG. 7 shows the serum clearance of human transferrin alone, sdAb A60219 alone, and sdAb A60219 after injection into a Wister rat supplemented with human transferrin; a sample of blood was taken at the indicated timepoints and the concentrations of A60219 and human transferrin in the blood sample were determined by enzyme linked immunosorbent assay (ELISA) in three experimental groups.

DETAILED DESCRIPTION OF THE INVENTION

Various publications are cited or described in the background and throughout the specification and each of these references is herein incorporated by reference in its entirety. Discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is for the purpose of providing context for the present invention. Such discussion is not an admission that any or all of the matters form part of the prior art with respect to any inventions disclosed or claimed.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this invention pertains. Otherwise, certain terms used herein have the meanings as set forth in the specification. It must be noted that as used herein and in the appended claims, the singular forms “a,” “an,” and “the” include plural reference unless the context clearly dictates otherwise.

One of ordinary skill in the art will be familiar with the structure of an antibody. The light and heavy chains each contain a variable region that is responsible for binding the target antigen. The variable region contains the antigen binding determinants of the molecule, thus determining the specificity of an antibody for its target antigen. The variable regions of the light and heavy chains comprise three complementarity determining regions (CDRs).

As used herein “complementarity determining region” or “CDR” refers to an amino acid sequence of a variable region of a heavy or light chain of an antibody that contributes to specific recognition of, and binding specificity for, the antigen. The CDRs are referred to as CDR1, CDR2, and CDR3. According to embodiments of the present invention, at least one of the sequences of CDR1, CDR2, and CDR3 contributes to specific recognition of, and binding specificity for, an antibody or fragment thereof against transferrin, and preferably against human transferrin.

An “antibody fragment” as used herein includes any suitable antigen-binding antibody fragment. For example, an antibody fragment can comprise a single-chain variable region. According to embodiments of the present invention, an antibody is preferably a single-domain antibody (sdAb).

As used herein, “single-domain antibody” or “sdAb” refers to the antigen-binding site of a heavy-chain antibody (HCAb) of camelids and sharks, which is naturally devoid of light chains. Camelids include camel, llama, and alpaca. The antigen-binding site of HCAb of camelids is formed by a single variable domain designated VHH. The sdAbs usually exist as monomeric proteins having relatively small sizes. An sdAb according to the invention has three CDRs (CDR1, CDR2, and CDR3).

As used herein, “antibody or fragment thereof against transferrin,” “antibody or fragment thereof that specifically binds transferrin,” and “transferrin antibody,” shall all have the same meaning, and refer to an antibody or fragment thereof, that is capable of binding specifically to transferrin.

As used herein, “sdAb against transferrin,” “sdAb that specifically binds transferrin,” and “transferrin sdAb,” shall all have the same meaning, and refer to a single domain antibody that is capable of binding specifically to transferrin.

As used herein, “transferrin” broadly refers to a protein that binds iron. Transferrin, as used in the present invention, can be a transferrin from any organism that produces transferrin, and is more preferably a mammalian transferrin, such as human transferrin or monkey transferrin, and is most preferably a human transferrin.

As used herein, “binds specifically to” or “against” when used in connection with an antibody or fragment thereof and transferrin refers to the binding or interaction between the antibody and fragment thereof, such as an sdAb, and the transferrin. An antibody or fragment thereof, such as an sdAb, according to the invention binds to a transferrin with a dissociation constant (K_(D)) of between 10⁻⁶ and 10⁻⁹ M, or less, and preferably with a dissociation constant of less than 10⁻⁹ M, e.g., a dissociation constant in the nanomolar to picomolar range (10⁻⁹ to 10⁻¹²).

Any method known in the art can be used for determining specific antigen-antibody binding including, for example, surface plasmon resonance (SPR), scatchard analysis and/or competitive binding assays, such as radioimmunoassay (RIA), enzyme immunoassays (EIA) and sandwich competition assays, and the different variants thereof known in the art, as well as other techniques mentioned herein. Methods for determining the binding affinities or dissociation constants are known to those skilled in the art.

Any method known in the art can be used for characterizing an antibody or sdAb according to the invention, such as SDS-polyacrylamide gel electrophoresis (PAGE), circular dichroism (CD), size exclusion chromatography (SEC), etc. Methods for characterizing proteins, i.e. determining the oligomeric state, melting temperature, molecular weight, purity, etc., are known to those skilled in the art.

As used herein, the “half-life” of a protein or polypeptide refers to the time taken for the concentration of the polypeptide to be reduced by 50% in an assay conducted in vivo or in vitro. The reduction can be caused by degradation, clearance, or sequestration of the polypeptide in the assay. The half-life of a polypeptide can be determined by any manner known in the art in view of the present disclosure, such as by pharmacokinetic analysis. For example, to measure the half-life of a protein or polypeptide in vivo, a suitable dose of the polypeptide is administered to a warm-blooded animal (i.e. to a human or to another suitable mammal, such as a mouse, rabbit, rat, pig, dog, or a primate); blood samples or other samples from the animal are collected; the level or concentration of the protein or polypeptide in the sample is determined; and the time until the level or concentration of the polypeptide has been reduced by 50% compared to the initial level upon dosing is calculated based on measured data. See, e.g., Kenneth, A et al., Chemical Half-life of Pharmaceuticals: A Handbook for Pharmacists and Peters et al., Pharmacokinetic analysis: A Practical Approach (1996).

As used herein, “an increase in half-life” or “longer half-life” refers to an increase in any one of the parameters used to describe the protein half-life, such as the t½-α, t½β and the area under the curve (AUC), any two of these parameters, or essentially all of these parameters, as compared to a control.

As used herein, a “fusion tag” is a polypeptide sequence that can be operably linked to a target protein or polypeptide to generate a fusion protein for the ease of subsequent manipulation, such as for the expression, purification, in vitro and in vivo analysis and characterization of the protein, or diagnostic or therapeutic application. A fusion tag can exhibit one or more properties. For example, the fusion tag can selectively bind to a purification medium that contains a binding partner for the fusion tag and allows the operably linked target protein or fusion protein to be easily purified. The fusion tag can be, for example, glutathione S-transferase (GST), maltose binding protein, polyhistidine (His-tag), FLAG-tag, avidin, biotin, streptavidin, chitin binding domain, a ligand of a cellular receptor, the Fc region of an antibody, green fluorescent protein, etc.

The present invention relates to antibodies or fragments thereof against transferrin and methods of using antibodies or fragments thereof against a transferrin to increase the half-life of a target protein, i.e., by obtaining a fusion protein comprising a target protein and an antibody or fragment thereof against a transferrin, and exposing the fusion protein to the transferrin, for example, in a serum. The fusion protein can be exposed to the transferrin in a serum either in vitro or in vivo. In a particular embodiment, the invention relates to novel sdAbs against transferrin and their uses.

Accordingly, in one general aspect, the present invention provides an isolated antibody or fragment thereof that specifically binds a transferrin, the antibody or fragment thereof comprising one or more selected from the group consisting of:

-   -   (a) a complementarity determining region (CDR) 1 comprising an         amino acid sequence selected from the group consisting of         GSGFGINGVI (SEQ ID NO: 1); GSGFGVNGVI (SEQ ID NO: 2); GNVFTIAAMG         (SEQ ID NO: 3); GNVFTIAAMA (SEQ ID NO: 4); GNVFTIDAMG (SEQ ID         NO: 5); GSVFSIDAMG (SEQ ID NO: 6); GNVFGIDAVG (SEQ ID NO: 7);         GSIFSIKVMG (SEQ ID NO: 8); and GSIFPLNDMG (SEQ ID NO: 9);     -   (b) a CDR2 comprising an amino acid sequence selected from the         group consisting of LIKSDGYTNYRESVKG (SEQ ID NO: 10);         LIKSDGYTNYRESVRG (SEQ ID NO: 11); GITTGGSTNYADSVKG (SEQ ID NO:         12); GMTNGGKTNYADSVKG (SEQ ID NO: 13); AMTNAGSTNYADSVKG (SEQ ID         NO: 14); ATTTSGSSTNYADSVKG (SEQ ID NO: 15); DITSGGSTDYSDSVKG         (SEQ ID NO: 16); and TITRGGTTNYADSVKG (SEQ ID NO: 17); and     -   (c) a CDR3 having comprising an amino acid sequence selected         from the group consisting of PGVP (SEQ ID NO: 18);         VTKWAARVGGSAEYE (SEQ ID NO: 19); RSKLIATINNPYDY (SEQ ID NO: 20);         RSKLIARINNPYEY (SEQ ID NO: 21); RPKQATLIRDDY (SEQ ID NO: 22);         DLGCSGAGSCPDY (SEQ ID NO: 23); and DNRVGGSY (SEQ ID NO: 24).

According to embodiments of the present invention, an isolated antibody, preferably an sdAb, or fragment thereof, comprises a CDR 1 selected from the group consisting of SEQ ID NOs: 1-9, a CDR 2 selected from the group consisting of SEQ ID NOs: 10-17, and a CDR 3 selected from the group consisting of SEQ ID NOs: 18-24.

According to embodiments of the present invention, an isolated antibody or fragment thereof that specifically binds a transferrin has an affinity (K_(D)) for transferrin that is between 10⁻⁶ and 10⁻⁹M or less, and preferably has an affinity lower than 10⁻⁹ M. In a most preferred embodiment, an isolated antibody or fragment thereof according to the invention has a K_(D) for transferrin that is in the subnanomolar range, for example, in the picomolar range, such as 1-10 pM, 15 pM, 20 pM, 30 pM, 40 pM, 50 pM, 60 pM, 70 pM, 80 pM, 90 pM, or 100 pM.

In a particular embodiment, an isolated antibody or fragment thereof according to an embodiment of the present invention comprises a CDR1 amino acid sequence of SEQ ID NO: 1, a CDR2 amino acid sequence of SEQ ID NO: 10, and a CDR3 amino acid sequence of SEQ ID NO: 18. In yet another particular embodiment, an isolated antibody or fragment thereof according to the present invention comprises a CDR1 amino acid sequence of SEQ ID NO: 3, a CDR2 amino acid sequence of SEQ ID NO: 12, and a CDR3 amino acid sequence of SEQ ID NO: 19. In other particular embodiments, an isolated antibody or fragment thereof according to the present invention comprises a CDR1 amino acid sequence of SEQ ID NO: 6, a CDR2 amino acid sequence of SEQ ID NO: 14, and a CDR3 amino acid sequence of SEQ ID NO: 21; a CDR1 amino acid sequence of SEQ ID NO: 7, a CDR2 amino acid sequence of SEQ ID NO: 15, and a CDR3 amino acid sequence of SEQ ID NO: 22; or a CDR1 amino acid sequence of SEQ ID NO: 9, a CDR2 amino acid sequence of SEQ ID NO: 17, and a CDR3 amino acid sequence of SEQ ID NO: 24.

According to embodiments of the present invention, an antibody or fragment thereof, such as an sdAb, that specifically binds a transferrin can comprise an amino acid sequence selected from the group consisting of SEQ ID NOs: 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60, and 62.

According to a preferred embodiment of the present invention, an antibody or fragment thereof that specifically binds transferrin is an sdAb. For example, an sdAb according to the invention that specifically binds transferrin can be a camelid V_(H)H antibody.

In a particularly preferred embodiment of the present invention, an sdAb that specifically binds transferrin comprises the

amino acid sequence of A60219: (SEQ ID NO: 26) QVQLVESGGGLVQAGGSLRLSCVASGSGFGINGVIWYRQAPGKQRELVAL IKSDGYTNYRESVKGRFTISRDDAKNTVWLQMNALEPEDTGVYYCKTPGV PFGQGTQVTVSS; the amino acid sequence of A60401: (SEQ ID NO: 28) QVKLEESGGGLVQAGGSLRLSCEASGNVFTIAAMGWFRQAPGKERELVAG ITTGGSTNYADSVKGRFTISRDNAQNTMYLQMNSLRPEDTAAYSCNAVTK WAARVGGSAEYEYWGQGTQVTVSS; the amino acid sequence of A69449: (SEQ ID NO: 56) QVQLVESGGGVVQAGGSLRLSCVASGSVFSIDAMGWYRQAPGNQRELVAA MTNAGSTNYADSVKGRFTISRDNAENTVYLQMNSLKPEDTAVYYCNARSK LIARINNPYEYWGQGTQVTVSS; the amino acid sequence of A69433: (SEQ ID NO: 58) QVKLEESGGGLVQAGGSLRLSCVASGNVFGIDAVGWYRQAPGKQRELVAA TTTSGSSTNYADSVKGRFTISRDIAKNTVYLQMDSLKPEDTAVYYCYARP KQATLIRDDYWGQGTQVTVSS; or the amino acid sequence of A69447: (SEQ ID NO: 62) QVKLEESGGGSVQAGGSLRLSCTGSGSIFPLNDMGWYRQAPGKQRELVAT ITRGGTTNYADSVKGRFTISRDSNAKNTVYLQMNSLKVEDTAVYYCNMDN RVGGSYWGQGTQVTVSS.

The present invention also provides a nucleic acid comprising a complementary DNA (cDNA) sequence encoding an antibody or fragment thereof according to an embodiment of the invention. Also provided are vectors comprising the nucleic acid molecule, particularly expression vectors, and recombinant host cells comprising the vectors that can subsequently be used for downstream applications such as expression, purification, etc. The nucleic acid molecules, vectors and host cells can be obtained using methods known in the art in view of the present disclosure.

An antibody or fragment thereof according to embodiments of the invention can be produced recombinantly from a recombinant host cell using methods known in the art in view of the present disclosure. The recombinantly produced antibody or fragment thereof can be different from the naturally occurring antibody or fragment thereof, for example, in posttranslational modification of amino acids. As used herein, “posttranslational modification of amino acids” refers to any modification to the amino acids after translation of the amino acids, such as by attaching to one or more amino acids independently one or more biochemical functional groups (such as acetate, phosphate, various lipids and carbohydrates), changing the chemical nature of an amino acid (e.g. citrullination), or making structural changes (e.g. formation of disulfide bridges).

According to embodiments of the present invention, a nucleic acid molecule comprises a cDNA or synthetic DNA sequence encoding an amino acid sequence selected from the group consisting of SEQ ID NOs: 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60, and 62. In particular embodiments of the present invention, the nucleic acid molecule comprises a cDNA or synthetic DNA sequence comprising a sequence selected from the group consisting of SEQ ID NOs: 25, 27, 29, 31, 33, 35, 37, 39, 41, 43, 45, 47, 49, 51, 53, 55, 57, 59 and 61. However, one skilled in the art will recognize that the specific nucleotide sequences listed are intended to be non-limiting examples, and various nucleotide sequences can encode the same amino acid sequence.

As used herein, a “cDNA or synthetic DNA” refers to a DNA molecule that is different from a naturally occurring DNA molecule in at least one of the nucleotide sequence and the physical or chemical property of the DNA molecule. For example, the “cDNA or synthetic DNA” can be different from the naturally occurring DNA in nucleotide sequence by not containing one or more introns present in the natural genomic DNA sequence. The “cDNA or synthetic DNA” can also be different from a naturally occurring DNA in one or more physical or chemical properties, such as having a different DNA modification, regardless of whether the “cDNA or synthetic DNA” comprises the same or different nucleotide sequence as that of the naturally occurring DNA.

As used herein, “DNA modification” refers to any modification to the DNA, such as by independently attaching to one or more nucleotides of the DNA one or more biochemical functional groups (such as a methyl group, phosphate group, etc). Different host cells can have different DNA modification systems, thus producing different DNA molecules even though the DNA molecules can have identical nucleotide sequence.

A “cDNA or synthetic DNA” can be made by any method in vivo or in vitro so long as the obtained “cDNA or synthetic DNA” is distinguishable from a naturally occurring DNA molecule. For example, a “cDNA” can be made from a messenger RNA (mRNA) template in a reaction catalyzed by the enzymes reverse transcriptase and DNA polymerase, or RNA-dependent DNA polymerase. In one embodiment, a “cDNA” can be made and amplified via a reverse transcriptase polymerase chain reaction (RT-PCR) with the desired mRNA template and DNA primers. A “synthetic DNA” can be made in vitro using any method known in the art. A “synthetic DNA” can also be made in vivo in a host cell that does not naturally contain a nucleic acid molecule having the identical nucleotide sequence as that of the “synthetic DNA,” such that the “synthetic DNA” made by the host cell is distinguishable from any naturally occurring DNA sequence in at least one or more physical or chemical properties, such as DNA methylation.

Embodiments of the present invention also relate to methods of recombinantly expressing and purifying an antibody or fragment thereof that specifically binds transferrin. According to embodiments of the present invention, the method comprises obtaining an expression vector encoding an antibody or fragment thereof according to an embodiment of the invention, introducing the expression vector into a host cell to obtain a recombinant cell, growing the recombinant cell under conditions that allow expression of the antibody or fragment thereof, and obtaining the antibody or fragment thereof from the recombinant cell. The antibody or fragment thereof that specifically binds transferrin can be isolated by applying the lysate, supernatant, or periplasmic extract of the recombinant cell comprising the antibody or fragment thereof, to an affinity column associated with the transferrin.

In another embodiment, the antibody or fragment thereof that specifically binds a transferrin further comprises a fusion tag that facilitates purification of the antibody or fragment thereof from the recombinant cell, by for example, applying the lysate, supernatant, or periplasmic extract of the recombinant cell comprising of the antibody or fragment thereof, to an affinity column associated with a binding partner of the fusion tag. As an illustrative and non-limiting example, a fusion tag can be a 6×-HIS tag, and the 6×-HIS tagged antibody or fragment thereof can be purified from the recombinant cell by applying the lysate, supernatant, or periplasmic extract of the recombinant cell to a nickel column.

Antibodies or fragments thereof, and particularly sdAbs, according to embodiments of the invention that specifically bind transferrin have high affinity for transferrin (e.g., picomolar range) and a longer half-life in a serum supplemented with transferrin, as compared to the half-life in a serum that is not supplemented with the transferrin. Without wishing to be bound by theory, it is believed that the binding interaction between the antibody and fragment thereof, such as sdAb, and the transferrin contributes to the longer half-life of the antibody or fragment thereof in a serum. Such antibody or fragment thereof can thus be used to increase the half-life of a target protein fused to the antibody or fragment thereof in a serum, or in a composition comprising the transferrin.

Thus, in another general aspect, the present invention relates to a method of increasing the half-life of a target protein in a serum. The method comprises (1) obtaining a fusion protein, wherein the fusion protein comprises an antibody or fragment thereof that specifically binds a transferrin, the target protein, and optionally a linker, wherein the antibody or fragment thereof is fused to the carboxyl-terminus or amino-terminus of the target protein, and the linker optionally separates the antibody and the carboxyl-terminus or amino-terminus of the target protein; and (2) exposing the fusion protein to the transferrin to thereby increase the half-life of the target protein in the fusion protein compared to the target protein alone.

According to embodiments of the present invention, the transferrin used in the exposing step can be present in any composition, including a serum or a buffered composition made in vitro. Preferably, the fusion protein is exposed to the transferrin by administering it to a serum comprising the transferrin.

According to embodiments of the present invention, when exposed to the transferrin, the fusion protein has an increased half-life, as determined, for example by measuring the half-life, as compared to the target protein alone. The fusion protein can optionally contain a linker that fuses the target protein to the antibody or fragment thereof, and also functions to separate the target protein from the antibody or fragment thereof. Linkers that can be used to fuse two protein molecules together will be well known to those skilled in the art in view of the present disclosure.

The antibody or fragment thereof can be fused to the target protein by any method known in the art in view of the present disclosure, such as, for example, via genetic fusion or covalent linkage. The antibody or fragment thereof can be fused to either the amino-terminus or the carboxyl-terminus of the target protein.

Preferably, the fusion protein comprises an antibody or fragment thereof according to embodiments of the present invention. For example, the antibody or fragment thereof comprises an amino acid sequence selected from the group consisting of the amino acid sequence of CDR1 comprising the sequence of SEQ ID NO: 1, 2, 3, 4, 5, 6, 7, 8, or 9; the amino acid sequence of CDR2 comprising the sequence of SEQ ID NO: 10, 11, 12, 13, 14, 15, 16, or 17; and the amino acid sequence of CDR3 comprising the sequence of SEQ ID NO: 18, 19, 20, 21, 22, 23, or 24. Most preferably, the antibody or fragment thereof of the fusion protein is an sdAb and even more preferably is an sdAb comprising the amino acid sequence of A60219 (SEQ ID NO: 26), A60401 (SEQ ID NO: 28), A69449 (SEQ ID NO: 56), A69433 (SEQ ID NO: 58), or A69447 (SEQ ID NO: 62).

According to embodiments of the present invention, the target protein is a peptide or polypeptide, such as a therapeutic polypeptide, a polypeptide that can be used for a diagnostic purpose, or a polypeptide for structural-activity studies. For example, the target protein can be an antibody, peptide, or any other polypeptide that has been or will be developed or used for a therapeutic or diagnostic purpose, or a protein subjected to structural and/or functional analysis. Preferably, the target protein is a therapeutic peptide or polypeptide that is unstable in serum, and in need of increased serum half-life to be used for therapeutic, diagnostic purposes, etc.

The fusion proteins according to embodiments of the present invention can be used for various purposes using methods known in the art in view of the present disclosure. For example, a fusion protein can be used for assaying the affinity of the target protein to a binding partner, e.g., for drug screening or target identification purposes. It can also be used in a diagnostic method, particularly if the method involves administering the target protein to the serum. It can further be used for therapeutic purposes, particularly if the target protein is known to be unstable in serum.

Another general aspect of the invention relates to a composition comprising an effective amount of a fusion protein, wherein the fusion protein comprises an antibody or fragment thereof that specifically binds a transferrin, a target protein, and optionally a linker, wherein the antibody or fragment thereof is fused to the carboxyl-terminus or amino-terminus of the target protein, and the linker optionally separates the antibody and the carboxyl-terminus or amino-terminus of the target protein. Preferably, the composition further comprises the transferrin, which increases the half-life of the fusion protein in the composition. The composition can further comprise a pharmaceutically acceptable carrier, which can comprise any carrier that is suitable for pharmaceutical or diagnostic purposes.

Depending on the use, the effective amount can be the amount of the fusion protein that is effective to provide a therapeutic or diagnostic use of the target protein as part of the fusion. For example, an effective amount of the fusion protein, when used for a therapeutic application, can be the amount fusion protein that treats and/or prevents a disease, disorder, or condition treatable or preventable by the target protein as part of the fusion protein. An effective amount of the fusion protein, when used for a diagnostic application, can be the amount of the fusion protein needed to detect a marker, e.g., biomolecule such as nucleic acid or protein, by the target protein as part of the fusion protein.

A composition according to an embodiment of the present invention can be used in vivo or in vitro for any purpose. Compositions of the invention for in vivo uses can be formulated for any method of delivery to a subject, e.g., a mammal, such as human, rat, mice, monkey, or rabbit, including, but not limited to oral, topical, and injection. Preferably a composition for in vivo use is formulated for injection or intravenous administration.

The present invention also relates to a method comprising exposing a composition according to an embodiment of the present invention to the transferrin to thereby increase the half-life of the target protein in the fusion protein.

In one embodiment, the present invention relates to a method comprising exposing a composition according to an embodiment of the present invention to the transferrin used in the fusion protein, for example, by administering the composition to a serum comprising transferrin, in vivo or in vitro for identifying a diagnostic or therapeutic agent.

In another embodiment, a method according to the present invention comprises administering a composition according to an embodiment of the present invention to a subject in need of treatment by the target protein, wherein the composition comprises a therapeutically effective amount of a fusion protein comprising an antibody or fragment thereof that specifically binds the human transferrin and a target protein.

In yet another embodiment, a method according to the invention comprises administering the composition to a subject in need of a diagnosis by the target protein, wherein the composition comprises a diagnostically effective amount of the fusion protein.

Embodiments of the present invention also relate to compositions comprising a fusion protein according to the invention, and methods for increasing the half-life of a target protein in a composition. A method for increasing the half-life of a target protein in a composition comprises obtaining a fusion protein, preferably isolated fusion protein, comprising an antibody or fragment thereof against a transferrin, the target protein and an optional linker, wherein the antibody or fragment thereof is fused to the carboxyl-terminus or amino-terminus of the target polypeptide, and the optional linker separates the antibody or fragment thereof and the carboxyl-terminus or amino-terminus of the target polypeptide; and exposing the fusion protein to the transferrin in the composition, wherein the fusion protein has a longer half-life than the target protein alone in the composition.

Without wishing to be bound by theory, it is believed that the specific binding between the antibody or fragment thereof in the fusion protein and the transferrin in a composition or a serum contributes to increased half-life of the target protein.

Embodiments of the present invention also provides methods for obtaining a target protein having increased serum half-life, and for expressing and purifying a fusion protein comprising an antibody or fragment thereof that binds to a transferrin and a target protein.

According to embodiments of the present invention, a method for obtaining a target protein having increased serum half-life comprises:

-   -   (a) obtaining an expression vector encoding a fusion protein         comprising an antibody or fragment thereof that specifically         binds a transferrin, the target protein, and an optional linker,         wherein the antibody or fragment thereof is fused to the         carboxyl-terminus or amino-terminus of the target protein, and         the optional linker separates the antibody and the         carboxyl-terminus or amino-terminus of the target protein;     -   (b) introducing the expression vector of step (a) into a cell to         obtain a recombinant cell;     -   (c) growing the recombinant cell under conditions to allow         expression of the fusion protein; and     -   (d) obtaining the fusion protein from the recombinant cell.

Expression vectors encoding the fusion protein and recombinant cells expressing the fusion protein can be constructed using methods known in the art in view of the present disclosure. Any host cell suitable for recombinant production of the fusion protein can be used such as a mammalian cell, plant cell, yeast cell, or bacterial cell. Preferably, the host cell is a bacterial cell, and is more preferably Escherichia coli. Any method for obtaining the fusion protein from the recombinant cell can be used in view of the present disclosure including, but not limited to, column chromatography such as affinity chromatography.

In one embodiment, the fusion protein can be obtained from a recombinant cell and purified by utilizing the specific interaction between the portion of the fusion protein comprising the antibody or fragment thereof that specifically binds transferrin, and transferrin, to obtain the fusion protein from the recombinant cell, e.g., by affinity chromatography using an affinity column associated with transferrin.

In another embodiment, the fusion protein can further comprise a fusion tag at the amino-terminus or carboxyl-terminus of the fusion protein to facilitate obtaining and purifying the fusion protein from the recombinant cell. The fusion tag can be fused to the transferrin, or to the antibody or fragment thereof that binds transferrin. For example, the lysate, periplasmic extract, or supernatant of the recombinant cell comprising the fusion protein can be obtained and applied to a column associated with the appropriate binding partner of the fusion tag. In a particular and non-limiting example, the fusion protein can further comprise a His-tag and the lysate, periplasmic extract, or supernatant of the recombinant cell comprising the fusion protein can be obtained and applied to a nickel column to obtain the fusion protein from the recombinant cell. The column can then be washed, and the fusion protein eluted from the column under the appropriate buffering conditions to obtain the fusion protein.

Another general aspect of the present invention relates to a system for increasing the half-life of a target protein, comprising:

-   -   (1) an expression vector comprising a first nucleotide sequence         encoding an antibody or fragment thereof that specifically binds         a transferrin, and optionally a second nucleotide sequence         encoding a linker, wherein the first and second nucleotide         sequences are operably linked;     -   (2) a host cell; and     -   (3) the transferrin.

The expression vector can be used to construct an expression vector for a fusion protein comprising an antibody or fragment thereof that specifically binds a transferrin, a target protein, and optionally a linker, wherein the antibody or fragment thereof is fused to the carboxyl-terminus or amino-terminus of the target protein, and the linker optionally separates the antibody and the carboxyl-terminus or amino-terminus of the target protein. Thus, in certain embodiments of the invention, an expression vector comprises a first nucleotide sequence encoding an antibody or fragment thereof that specifically binds a transferrin, optionally a second nucleotide sequence encoding a linker, and a third nucleotide sequence encoding a target protein, wherein the first, second, and third nucleotide sequences are operably linked, such that the antibody or fragment thereof is fused to the carboxyl-terminus or amino-terminus of the target protein, and the linker optionally separates the antibody and the carboxyl-terminus or amino-terminus of the target protein.

The host cell can be used to construct a recombinant cell for expressing the fusion protein, e.g., by transforming the host cell with the expression vector for the fusion protein, using any method known in the art in view of the present invention, including but not limited to electroporation and calcium chloride transformation.

The transferrin can be used to stabilize the fusion protein. It can also be used to isolate the fusion protein by affinity chromatography.

According to an embodiment of the present invention, the system can further comprise a solid support for capturing the fusion protein via specific binding between the antibody or fragment thereof in the fusion protein and the transferrin associated with the solid support, or via specific binding between a fusion tag on the fusion protein and a binding partner of the fusion tag associated with the solid support.

The system can further comprise one or more buffers useful for the expression and/or isolation of the fusion protein.

The following specific examples of the invention are further illustrative of the nature of the invention, and it needs to be understood that the invention is not limited thereto.

EXAMPLES Example 1: Production and Characterization of sdAbs that Specifically Bind a Transferrin

Materials and Methods

Isolation of Transferrin sdAbs from a Llama Immune Phage Display Library

A male llama (Lama glama) was injected subcutaneously with 100 μg, 50 μg, 50 μg, 10 μg, and 10 μg human transferrin on days 1, 22, 36, 50 and 64, respectively [11]. Complete Freund's Adjuvant (Sigma, St. Louis, Mo.) was used for the primary immunization and Incomplete Freund's Adjuvant was used for subsequent immunizations 2-4. Adjuvant was not used for the final immunization. The llama was bled one week following each immunization and heparinized blood was collected for immediate isolation of the peripheral blood leukocytes, which were then stored at −80° C. until further use.

Total RNA was isolated from 1×10⁸ leukocytes using a QIAamp RNA Blood Mini Kit (Qiagen). cDNA was synthesized using pd(N)₆ as primer and 566 ng total RNA as the template. Four forward primers P441_VHHF1 (GCCCAGCCGGCCATGGCCSMBGTRCAGCTGGTGGAKTCTGGGGGA; SEQ ID NO: 63), P442_VHHF2 (GCCCAGCCGGCCATGGCCCAGGTAAAGCTGGAGGAGTCTGGGGGA; SEQ ID NO: 64), P759_VHHF3 (GCCCAGCCGGCCATGGCCCAGGTACAGCTGGTGGAGTCT; SEQ ID NO: 65) and P444_VHHF4 (GCCCAGCCGGCCATGGCCGAGGTGCAGCTGGTGGAGTGTGG; SEQ ID NO: 66); and two reverse primers P445_CH2R (CGCCATCAAGGTACCAGTTGA; SEQ ID NO: 67) and P446_CH2b3R (GGGGTACCTGTCATCCACGGACCAGCTGA SEQ ID NO: 68) were used to amplify V_(H)-C_(H)1-Hinge-C_(H)2 region of conventional immunoglobulin G antibody (IgG) or V_(H)H-Hinge-C_(H)2 of heavy chain antibody. Amplified V_(H)H products of approximately 600 bp from the primer combinations of P445_C_(H)2R with each of the forward primers P441_VHHF1, P442_VHHF2, P759_VHHF3, and P444_VHHF4 were extracted from a 1% agarose gel, and purified with a QIAquick Gel Extraction Kit (Qiagen) and the amplified products from primers P446_C_(H)2R were PCR purified. In a second PCR reaction, two primers, P440_VHHF (CATGTGTAGACTCGCGGCCCAGCCGGCCATGGCC; SEQ ID NO: 69) and P447_VHHR (CATGTGTAGATTCCTGGCCGGCCTGGCCTGAGGAGACGGTGACCTG; SEQ ID NO: 70) were used to introduce SfiI restriction sites, and to amplify the final sdAb fragments from the combined amplified products. The final PCR product was digested with SfiI and ligated into a conventional phagemid vector constructed at GenScript Inc., and transformed into E. coli TG1 by electroporation. Phage were rescued and amplified with helper phage M13KO7 (New England Biolabs (NEB)).

The llama immune phage display library was panned against transferrin that was conjugated to M-280 beads (Invitrogen). Approximately 3×10¹¹ phages were added to the beads and incubated at 37° C. for 2 hours for antigen binding. After disposal of unbound phages, the beads were washed six times with phosphate buffered saline supplemented with 0.05% Tween 20 (PB ST) for round one, and the washes were increased by one for each additional round. Phages were eluted by 10 min incubation with 100 μl 100 mM triethylamine, and the eluate was subsequently neutralized with 200 μl M Tris-HCl (pH 7.5). Phages were amplified as described above, but on a smaller scale. After two rounds of panning, eluted phages were used to infect exponentially growing E. coli TG1. Individual colonies were used in phage enzyme-linked immunosorbent assay (ELISA).

For phage ELISA, a 96-well microliter plate was coated overnight with 2 μg/ml human transferrin and then blocked with 4% modified phosphate buffered saline (MPBS) for 2 hours at 37° C. Phage from individual clones were pre-blocked with 4% MPBS overnight, added to the pre-blocked wells and incubated for 1 hour. Phage ELISA was performed using the GE Healthcare Detection Module Recombinant Phage Antibody System (GE Healthcare, Uppsala, Sweden) and positive phage clones were sequenced.

Expression of sdAbs

DNA encoding each sdAb (A60401, A60219, A69433, A69447, or A69449) (FIG. 2) was cloned into the BbsI and BamHI sites of a periplasmic expression vector pSJF2H [12], which added a 5× Histidine purification tag at the C-terminus of the sdAbs. These sdAbs were expressed periplasmically and purified by immobilized metal ion affinity chromatography (IMAC) [13]. Briefly, clones were inoculated in 25 ml LB-Ampicillin (Amp) and incubated at 37° C. with 200 rpm shaking overnight. The next day, 20 ml of the culture were used to inoculate 1 L of M9 medium (0.2% glucose, 0.6% Na₂HPO₄, 0.3% KH₂PO₄, 0.1% NH₄Cl, 0.05% NaCl, 1 mM MgCl₂, 0.1 mM CaCl₂)) supplemented with 0.4% casamino acids, 5 mg/L of vitamin B1, and 200 μg/ml of Amp, and cultured for 24 hours. 100 ml of 10×TB nutrients (12% Tryptone, 24% yeast extract and 4% glycerol), 2 ml of 100 mg/ml Amp, and 1 ml of 1 M isopropyl-beta-D-Thiogalactopyranoside (IPTG) were added to the culture and incubation was continued for another 65-70 hours at 28° C. with 200 rpm shaking. E. coli cells were harvested by centrifugation and lysed with lysozyme. Cell lysates were centrifuged, and clear supernatant was loaded onto High-Trap™ chelating affinity columns (GE Healthcare), and His-tagged proteins were purified.

Surface Plasmon Resonance (SPR) Analysis

Experiments were performed using a BIAcore T200 optical sensor platform and research grade CM5 sensor chips (GE Healthcare). Human transferrin was immobilized on the sensor chip surface by standard amine coupling. All experiments were carried out in HEPES buffer [10 mM HEPES (pH 7.4), 150 mM NaCl, 3.4 mM EDTA, 0.005% Tween 20] at 25° C. Antibodies were injected at serial dilutions ranging from 0.25 nM to 16 nM at a flow rate of 30 μl/min unless otherwise indicated. The amount of bound analyte after subtraction from the blank control surface is shown as relative response units (RU). The double referenced sensorgrams from each injection series were analyzed for binding kinetics using BIA evaluation software (GE Healthcare). Dissociation constants (K_(D)s) were calculated from the on- and off-rates (k_(on) and k_(off), respectively), as determined by global fitting of the experimental data to a 1:1 Langmuir binding model (Chi²<1).

Size Exclusion Chromatography

Size exclusion chromatography (SEC) analyses of sdAbs, A60401 and A60219, (FIG. 5) were performed with a Superdex 200™ column (GE Healthcare). Superdex separations were carried out in PBS. Low molecular weight (MW) markers ribonuclease A (13.7 kDa), chymotrypsin A (25 kDa) and ovalbumin (43 kDa) were used to calculate the MW of the sdAbs.

Measurement of Melting Temperature of sdAbs by Circular Dichroism

Proteins were separated with a Superdex75 SEC column in 10 mM phosphate buffer, pH 7.0. Peaks representing major components of proteins were collected and used in circular dichroism (CD) analysis. CD spectra were collected from 250 to 200 nm at protein concentrations of 100 ug/ml in a 10 mm quartz cuvette with a J-815 CD spectrometer (JASCO). CD spectra were measured at 2° C. intervals from 30° C. to 90° C. to determine thermal denaturation of proteins with a temperature shift speed of 1° C./min. Ellipticity at 202 nm, 208 nm, and 217 nm was plotted against temperature, and melting temperatures (T_(m)s) were calculated as the average T_(m) at the three wavelengths.

Measurement of Serum Half-Life

A group of three female Wister rats, each weighing approximately 250 g, were intravenously (i.v.) injected with 30 mg human transferrin, and 500 μg sdAb A60219, A60401, A69433, A69447, or A69449, respectively, immediately after injecting 30 mg human transferrin into the tail vein. Blood was collected from the eye through a glass capillary at indicated time points. Sera were separated and stored at −80° C. until further use. Concentrations of the injected antibody molecules in the above collected samples were measured by ELISA.

For the detection of transferrin, anti-transferrin antibody [HTF-14] (Abcam, ab769) was coated on microtitre plates (Costar, 9018) overnight at 4° C. at a concentration of 1 μg/ml. After washing three times with PBST, plates were blocked with 1% BSA in PBST for two hours at 37° C. Diluted sera (1% BSA in 0.05% PBS-T used as diluent) were added to the wells and incubated at 37° C. for 2 hours. After washing four times with PBST, HRP labeled anti-transferrin antibody (0.1 μg/ml) (Abcam, ab9538) was added to the wells and incubated for another 1 hour. After washing the plate with PBST, the color was developed with TMB substrate for 10 minutes, and the reaction was stopped by adding 1 M HCl. The absorbance of each well was measured at 450 nm using a spectrometer. Serial dilutions of pure human transferrin in 1% BSA in PBST were used to generate a standard curve for human transferrin concentration analysis.

The same method was used for the detection of sdAbs, except anti-sdAb rabbit polyclonal antibody (GenScript) was used as a capture antibody, and HRP labeled anti-sdAb rabbit polyclonal antibody (0.1 μg/ml) (GenScript) was used as detection antibody. Serial dilutions of pure sdAbs in 1% BSA in PBST were used to make a standard curve for the concentration of the sdAbs.

Results

Isolation and Characterization of sdAbs

Isolation of transferrin-specific sdAbs was achieved by llama immunization with human transferrin, construction of an immune phage display library from the llama, and subsequent panning.

Human transferrin induced a medium immune response in the llama. An approximately 25,000 fold dilution of the serum after the fifth immunization still detected positive (FIG. 1). This level of response is in agreement with what is usually achieved with llama immunization.

Approximately 1×10⁸ llama leukocytes were used for the isolation of mRNA, which was then used for the construction of a phage library. The size of the obtained library was 2×10⁸ independent transformants with a positive insertion rate of 92%. Two rounds of phage display panning were performed on immobilized transferrin, and phage enrichment was observed during panning (data not shown). Phage ELISA showed that approximately 88% of the analyzed clones bound to transferrin. Analysis of encoding sequences of the sdAbs displayed on the phage clones revealed 19 different sdAb amino acid sequences (SEQ ID NOs: 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60 and 62, and FIG. 2).

Five sdAbs, A60219, A60401, A69433, A69447 and A69449, were sub-cloned, respectively, into an E. coli periplasmic expression vector, pSJF2H [12]. The sdAbs, tagged with a 6× histidine (His) tag at their C-terminal ends, were then produced in E. coli and purified by IMAC (FIGS. 3A and 3B). The sdAb yields of A60219, A60401, A69433, A69447 and A69449 were 8.4, 8.5, 12.5, 17.2, and 7.8 mg per liter of TG1 culture, respectively.

The five transferrin sdAbs, A60219, A60401, A69433, A69447 and A69449, were analyzed for binding to transferrin using an SPR-based biosensor (FIG. 4). Among them, A69433, A69447 and A69449, showed excellent cross binding activities with human and monkey transferrins (Table 2).

The results are shown in Table 1 and Table 2 below, and in FIG. 4. For example, the on-rates of the sdAbs were 9.27×10⁵ and 5.14×10⁷ M⁻¹s⁻¹, and the off-rates 6.22×10⁻⁵ and 6.74×10⁻⁴ s⁻¹ for A60401 and A60219, respectively. The dissociation constants (K_(D)s) of the sdAbs were calculated as 67.14 pM for A60401 (FIG. 4A) and 13.1 pM for A60219 (FIG. 4B).

TABLE 1 Transferrin sdAbs (A60219 and A60401) binding to human transferrin measured using SPR. Antibody ID Antigen k_(on) (1/Ms) k_(off) (1/s) K_(D) (M) A60219 Human transferrin 5.14E+07 6.74E−04 1.31E−11 A60401 Human transferrin 9.27E+05 6.22E−05 6.71E−11

TABLE 2 Transferrin sdAbs (A69433, A69447 and A69449) binding to human and monkey transferrins measured using SPR. Antibody ID Antigen k_(on) (1/Ms) k_(off) (1/s) K_(D) (M) A69433 Human Transferrin 1.11E+05 6.42E−04 5.76E−09 A69447 2.03E+05 2.09E−04 1.03E−09 A69449 3.42E+05 6.95E−04 2.03E−09 A69433 Monkey Transferrin 2.47E+05 5.93E−04 2.40E−09 A69447 3.04E+05 7.58E−05 2.50E−10 A69449 4.71E+05 5.29E−04 1.12E−09

Characterization of the sdAbs

Size exclusion chromatography (SEC) was employed to evaluate whether the two sdAbs A60401 and A60219 existed as monomers (FIG. 5). As with most camelid sdAbs, A60401 was a pure monomer, which was demonstrated as a single peak at 14.28 ml of elution volume through size exclusion chromatography with a Superdex75 column, corresponding to a measured MW of 10.6 kDa, very close to its calculated MW of 14,347 Da. A60219 very likely exists as pure monomer as well, as it has a single elution peak at 13.98 ml, representing an estimated MW of 11.2 kDa. However, its slightly faster elution from the column than A60401 indicates that dynamic transfer between monomeric and oligomeric forms of A60219 may exist, as it eluted faster than A60401 even with a smaller calculated MW (13,170 Da).

CD profiles of A60219 and A60401 were measured to estimate their secondary structures and thermo half-life. Both proteins had a CD profile that is typical for single domain antibodies (Data not shown). Thermo-induced protein denaturation was measured in the temperature range from 30 to 90° C. at 2° C. intervals. Plotting the CD values at 202 nm, 208 nm, and 217 nm against temperature suggested a two phase denaturation for both A60219 (FIG. 6A) and A60401 (FIG. 6B) with a calculated melting temperature (T_(m)) of 69.8° C. and 58.7° C., respectively.

Serum Clearance of sdAb A60219

A60219 was selected to test its serum half-life based on its high affinity, pure monomeric status, and high thermo half-life. A60219 was first injected into rat either by itself or immediately after injection of human transferrin into the rats. Human transferrin was also injected to measure its serum half-life in rats, mimicking an environment of existing human transferrin in the blood. An ELISA system was employed to measure the concentrations of A60219 in rat blood taken at different time points after injection.

Blood clearance of human transferrin in Wister rats was first investigated. After injecting 30 mg human transferrin into the rats, blood samples were collected for seven days. A gradual and steady decline of transferrin concentration was observed. The t_(1/2β was calculated as) 22 hours. This is significantly shorter than the serum half-life of human transferrin in human. Nevertheless, infusion of human transferrin in rats provides a model for us to estimate the serum half-life of A60219 in human. It is noteworthy that the anti-human transferrin antibody has no cross reactivity with rat transferrin, which makes the analysis of the serum clearance of the sdAb simple.

The serum half-life of the sdAb A60219 was then investigated. As many other sdAbs ever studied in animal models, A60219 was cleared from the rat blood rapidly. At approximately 8 hours only trace amounts of injected A60219 could be detected in the rat blood.

A60219 half-life was significantly longer in rats supplemented with human transferrin. Its blood clearance rate is very similar to that of human transferrin. The calculated serum half-life is also 22 hours. This indicates that, when injected into human, A60219 would have a similar serum half-life as human transferrin.

Results from the serum clearance study demonstrated that, exposing a polypeptide comprising an antibody or fragment thereof against a transferrin to the transferrin significantly increased the serum half-life of the polypeptide in vivo.

REFERENCES

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We claim:
 1. An isolated single-domain antibody (sdAb) or antigen-binding fragment thereof that specifically binds a transferrin, wherein the sdAb or antigen-binding fragment thereof comprises a complementarity determining region 1 (CDR1), CDR2, and CDR3 having the amino acid sequences, respectively, of: (a) SEQ ID NO: 1, SEQ ID NO: 10, and SEQ ID NO: 18; (b) SEQ ID NO: 3, SEQ ID NO: 12, and SEQ ID NO: 19; (c) SEQ ID NO: 6, SEQ ID NO: 14, and SEQ ID NO: 21; (d) SEQ ID NO: 7, SEQ ID NO: 15, and SEQ ID NO: 22; or (e) SEQ ID NO: 9, SEQ ID NO: 17, and SEQ ID NO:
 24. 2. The isolated sdAb or antigen-binding fragment thereof of claim 1, comprising the CDR1, CDR2, and CDR3 having the amino acid sequences, respectively, of SEQ ID NO: 3, SEQ ID NO: 12, and SEQ ID NO:
 19. 3. The isolated sdAb or antigen-binding fragment thereof of claim 1, comprising the CDR1, CDR2, and CDR3 having the amino acid sequences, respectively, of SEQ ID NO: 6, SEQ ID NO: 14, and SEQ ID NO:
 21. 4. The isolated sdAb or antigen-binding fragment thereof of claim 1, comprising the CDR1, CDR2, and CDR3 having the amino acid sequences, respectively, of SEQ ID NO: 7, SEQ ID NO: 15, and SEQ ID NO:
 22. 5. The isolated sdAb antibody or antigen-binding fragment thereof of claim 1, comprising the CDR1, CDR2, and CDR3 having the amino acid sequences, respectively, of SEQ ID NO: 9, SEQ ID NO: 17, and SEQ ID NO:
 24. 6. The isolated sdAb or antigen-binding fragment thereof of claim 1, comprising the amino acid sequence selected from the group consisting of: A60219: (SEQ ID NO: 26) QVQLVESGGGLVQAGGSLRLSCVASGSGFGINGVIWYRQAPGKQRELVAL IKSDGYTNYRESVKGRFTISRDDAKNTVWLQMNALEPEDTGVYYCKTPGV PFGQGTQVTVSS; A60401: (SEQ ID NO: 28) QVKLEESGGGLVQAGGSLRLSCEASGNVFTIAAMGWFRQAPGKERELVAG ITTGGSTNYADSVKGRFTISRDNAQNTMYLQMNSLRPEDTAAYSCNAVTK WAARVGGSAEYEYWGQGTQVTVSS; A69449: (SEQ ID NO: 56) QVQLVESGGGVVQAGGSLRLSCVASGSVFSIDAMGWYRQAPGNQRELVAA MTNAGSTNYADSVKGRFTISRDNAENTVYLQMNSLKPEDTAVYYCNARSK LIARINNPYEYWGQGTQVTVSS; A69433: (SEQ ID NO: 58) QVKLEESGGGLVQAGGSLRLSCVASGNVFGIDAVGWYRQAPGKQRELVAA TTTSGSSTNYADSVKGRFTISRDIAKNTVYLQMDSLKPEDTAVYYCYARP KQATLIRDDYWGQGTQVTVSS; and A69447: (SEQ ID NO: 62) QVKLEESGGGSVQAGGSLRLSCTGSGSIFPLNDMGWYRQAPGKQRELVAT ITRGGTTNYADSVKGRFTISRDSNAKNTVYLQMNSLKVEDTAVYYCNMDN RVGGSYWGQGTQVTVSS.


7. The isolated sdAb antibody or antigen-binding fragment thereof of claim 1, having a dissociation constant (K_(d)) of 10⁻⁹ M or less for the transferrin.
 8. The isolated sdAb antibody or antigen-binding fragment thereof of claim 1, wherein the transferrin is a human transferrin or monkey transferrin.
 9. The isolated sdAb antibody or antigen-binding fragment thereof of claim 1, being produced recombinantly.
 10. An isolated single-domain antibody (sdAb) antibody or antigen-binding fragment thereof that specifically binds a transferrin, wherein the sdAb or antigen-binding fragment thereof comprises a complementarity determining region 1 (CDR1), CDR2, and CDR3 having the amino acid sequences, respectively, of SEQ ID NO: 1, SEQ ID NO: 10, and SEQ ID NO:
 18. 11. The sdAb antibody or antigen-binding fragment thereof of claim 10, wherein the sdAb or antigen-binding fragment thereof comprises the amino acid sequence of SEQ ID NO:
 26. 12. The sdAb antibody or antigen-binding fragment thereof of claim 10, having a dissociation constant (K_(d)) of 10⁻⁹ M or less for the transferrin.
 13. The sdAb antibody or antigen-binding fragment thereof of claim 11, having a dissociation constant (K_(d)) of 10⁻⁹ M or less for the transferrin.
 14. The sdAb antibody or antigen-binding fragment thereof of claim 10, wherein the transferrin is a human transferrin or monkey transferrin.
 15. The sdAb antibody or antigen-binding fragment thereof of claim 11, wherein the transferrin is a human transferrin or monkey transferrin.
 16. The sdAb antibody or antigen-binding fragment thereof of claim 10, being produced recombinantly.
 17. The sdAb antibody or antigen-binding fragment thereof of claim 11, being produced recombinantly.
 18. A nucleic acid comprising a cDNA or synthetic DNA encoding the isolated sdAb antibody or antigen-binding fragment thereof of claim
 1. 19. An expression vector comprising the nucleic acid of claim
 18. 20. A recombinant host cell comprising the expression vector of claim
 19. 